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10 <title>Dynamic Shared Object (DSO) Support - Apache HTTP Server Version 2.5</title>
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20 <p class="apache">Apache HTTP Server Version 2.5</p>
21 <img alt="" src="./images/feather.png" /></div>
22 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
24 <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.5</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
26 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
27 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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33 <p>The Apache HTTP Server is a modular program where the
34 administrator can choose the functionality to include in the
35 server by selecting a set of modules.
36 Modules will be compiled as Dynamic Shared Objects (DSOs)
37 that exist separately from the main <code class="program"><a href="./programs/httpd.html">httpd</a></code>
38 binary file. DSO modules may be compiled at the time the server
39 is built, or they may be compiled and added at a later time
40 using the Apache Extension Tool (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p>
41 <p>Alternatively, the modules can be statically compiled into
42 the <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the server is built.</p>
44 <p>This document describes how to use DSO modules as well as
45 the theory behind their use.</p>
47 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#implementation">Implementation</a></li>
48 <li><img alt="" src="./images/down.gif" /> <a href="#usage">Usage Summary</a></li>
49 <li><img alt="" src="./images/down.gif" /> <a href="#background">Background</a></li>
50 <li><img alt="" src="./images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li>
51 </ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
52 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
54 <h2><a name="implementation" id="implementation">Implementation</a></h2>
56 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_so.html">mod_so</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code></li></ul></td></tr></table>
58 <p>The DSO support for loading individual Apache httpd modules is based
59 on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
60 compiled into the Apache httpd core. It is the only module besides
61 <code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
62 itself. Practically all other distributed Apache httpd modules will then
63 be placed into a DSO. After a module is compiled into a DSO named
64 <code>mod_foo.so</code> you can use <code class="module"><a href="./mod/mod_so.html">mod_so</a></code>'s <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directive in your
65 <code>httpd.conf</code> file to load this module at server startup
67 <p>The DSO builds for individual modules can be disabled via
68 <code class="program"><a href="./programs/configure.html">configure</a></code>'s <code>--enable-mods-static</code>
69 option as discussed in the <a href="install.html">install
70 documentation</a>.</p>
72 <p>To simplify this creation of DSO files for Apache httpd modules
73 (especially for third-party modules) a support program
74 named <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache
75 eXtenSion</dfn>) is available. It can be used to build DSO based
76 modules <em>outside of</em> the Apache httpd source tree. The idea is
77 simple: When installing Apache HTTP Server the <code class="program"><a href="./programs/configure.html">configure</a></code>'s
78 <code>make install</code> procedure installs the Apache httpd C
79 header files and puts the platform-dependent compiler and
80 linker flags for building DSO files into the <code class="program"><a href="./programs/apxs.html">apxs</a></code>
81 program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></code> to compile
82 his Apache httpd module sources without the Apache httpd distribution
83 source tree and without having to fiddle with the
84 platform-dependent compiler and linker flags for DSO
86 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
88 <h2><a name="usage" id="usage">Usage Summary</a></h2>
90 <p>To give you an overview of the DSO features of Apache HTTP Server 2.x,
91 here is a short and concise summary:</p>
95 <p>Build and install a <em>distributed</em> Apache httpd module, say
96 <code>mod_foo.c</code>, into its own DSO
97 <code>mod_foo.so</code>:</p>
99 <div class="example"><p><code>
100 $ ./configure --prefix=/path/to/install --enable-foo<br />
106 <p>Configure Apache HTTP Server with all modules enabled. Only a basic
107 set will be loaded during server startup. You can change the set of loaded
108 modules by activating or deactivating the <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives in
109 <code>httpd.conf</code>.</p>
111 <div class="example"><p><code>
112 $ ./configure --enable-mods-shared=all<br />
118 <p>Some modules are only useful for developers and will not be build.
119 when using the module set <em>all</em>. To build all available modules
120 including developer modules use <em>reallyall</em>. In addition the
121 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives for all
122 built modules can be activated via the configure option
123 <code>--enable-load-all-modules</code>.</p>
125 <div class="example"><p><code>
126 $ ./configure --enable-mods-shared=reallyall --enable-load-all-modules<br />
132 Build and install a <em>third-party</em> Apache httpd module, say
133 <code>mod_foo.c</code>, into its own DSO
134 <code>mod_foo.so</code> <em>outside of</em> the Apache httpd
135 source tree using <code class="program"><a href="./programs/apxs.html">apxs</a></code>:
137 <div class="example"><p><code>
138 $ cd /path/to/3rdparty<br />
139 $ apxs -cia mod_foo.c
144 <p>In all cases, once the shared module is compiled, you must
145 use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
146 directive in <code>httpd.conf</code> to tell Apache httpd to activate
149 <p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</p>
150 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
151 <div class="section">
152 <h2><a name="background" id="background">Background</a></h2>
154 <p>On modern Unix derivatives there exists a mechanism
155 called dynamic linking/loading of <em>Dynamic Shared
156 Objects</em> (DSO) which provides a way to build a piece of
157 program code in a special format for loading it at run-time
158 into the address space of an executable program.</p>
160 <p>This loading can usually be done in two ways: automatically
161 by a system program called <code>ld.so</code> when an
162 executable program is started or manually from within the
163 executing program via a programmatic system interface to the
164 Unix loader through the system calls
165 <code>dlopen()/dlsym()</code>.</p>
167 <p>In the first way the DSO's are usually called <em>shared
168 libraries</em> or <em>DSO libraries</em> and named
169 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
170 reside in a system directory (usually <code>/usr/lib</code>)
171 and the link to the executable program is established at
172 build-time by specifying <code>-lfoo</code> to the linker
173 command. This hard-codes library references into the executable
174 program file so that at start-time the Unix loader is able to
175 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
176 paths hard-coded via linker-options like <code>-R</code> or in
177 paths configured via the environment variable
178 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
179 unresolved) symbols in the executable program which are
180 available in the DSO.</p>
182 <p>Symbols in the executable program are usually not referenced
183 by the DSO (because it's a reusable library of general code)
184 and hence no further resolving has to be done. The executable
185 program has no need to do anything on its own to use the
186 symbols from the DSO because the complete resolving is done by
187 the Unix loader. (In fact, the code to invoke
188 <code>ld.so</code> is part of the run-time startup code which
189 is linked into every executable program which has been bound
190 non-static). The advantage of dynamic loading of common library
191 code is obvious: the library code needs to be stored only once,
192 in a system library like <code>libc.so</code>, saving disk
193 space for every program.</p>
195 <p>In the second way the DSO's are usually called <em>shared
196 objects</em> or <em>DSO files</em> and can be named with an
197 arbitrary extension (although the canonical name is
198 <code>foo.so</code>). These files usually stay inside a
199 program-specific directory and there is no automatically
200 established link to the executable program where they are used.
201 Instead the executable program manually loads the DSO at
202 run-time into its address space via <code>dlopen()</code>. At
203 this time no resolving of symbols from the DSO for the
204 executable program is done. But instead the Unix loader
205 automatically resolves any (yet unresolved) symbols in the DSO
206 from the set of symbols exported by the executable program and
207 its already loaded DSO libraries (especially all symbols from
208 the ubiquitous <code>libc.so</code>). This way the DSO gets
209 knowledge of the executable program's symbol set as if it had
210 been statically linked with it in the first place.</p>
212 <p>Finally, to take advantage of the DSO's API the executable
213 program has to resolve particular symbols from the DSO via
214 <code>dlsym()</code> for later use inside dispatch tables
215 <em>etc.</em> In other words: The executable program has to
216 manually resolve every symbol it needs to be able to use it.
217 The advantage of such a mechanism is that optional program
218 parts need not be loaded (and thus do not spend memory) until
219 they are needed by the program in question. When required,
220 these program parts can be loaded dynamically to extend the
221 base program's functionality.</p>
223 <p>Although this DSO mechanism sounds straightforward there is
224 at least one difficult step here: The resolving of symbols from
225 the executable program for the DSO when using a DSO to extend a
226 program (the second way). Why? Because "reverse resolving" DSO
227 symbols from the executable program's symbol set is against the
228 library design (where the library has no knowledge about the
229 programs it is used by) and is neither available under all
230 platforms nor standardized. In practice the executable
231 program's global symbols are often not re-exported and thus not
232 available for use in a DSO. Finding a way to force the linker
233 to export all global symbols is the main problem one has to
234 solve when using DSO for extending a program at run-time.</p>
236 <p>The shared library approach is the typical one, because it
237 is what the DSO mechanism was designed for, hence it is used
238 for nearly all types of libraries the operating system
241 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
242 <div class="section">
243 <h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2>
245 <p>The above DSO based features have the following
249 <li>The server package is more flexible at run-time because
250 the server process can be assembled at run-time via
251 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
252 <code>httpd.conf</code> configuration directives instead of
253 <code class="program"><a href="./programs/configure.html">configure</a></code> options at build-time. For instance,
254 this way one is able to run different server instances
255 (standard & SSL version, minimalistic & dynamic
256 version [mod_perl, mod_php], <em>etc.</em>) with only one Apache httpd
259 <li>The server package can be easily extended with
260 third-party modules even after installation. This is
261 a great benefit for vendor package maintainers, who can create
262 an Apache httpd core package and additional packages containing
263 extensions like PHP, mod_perl, mod_security, <em>etc.</em></li>
265 <li>Easier Apache httpd module prototyping, because with the
266 DSO/<code class="program"><a href="./programs/apxs.html">apxs</a></code> pair you can both work outside the
267 Apache httpd source tree and only need an <code>apxs -i</code>
268 command followed by an <code>apachectl restart</code> to
269 bring a new version of your currently developed module into
270 the running Apache HTTP Server.</li>
273 <p>DSO has the following disadvantages:</p>
276 <li>The server is approximately 20% slower at startup time
277 because of the symbol resolving overhead the Unix loader now
280 <li>The server is approximately 5% slower at execution time
281 under some platforms, because position independent code (PIC)
282 sometimes needs complicated assembler tricks for relative
283 addressing, which are not necessarily as fast as absolute
286 <li>Because DSO modules cannot be linked against other
287 DSO-based libraries (<code>ld -lfoo</code>) on all platforms
288 (for instance a.out-based platforms usually don't provide
289 this functionality while ELF-based platforms do) you cannot
290 use the DSO mechanism for all types of modules. Or in other
291 words, modules compiled as DSO files are restricted to only
292 use symbols from the Apache httpd core, from the C library
293 (<code>libc</code>) and all other dynamic or static libraries
294 used by the Apache httpd core, or from static library archives
295 (<code>libfoo.a</code>) containing position independent code.
296 The only chances to use other code is to either make sure the
297 httpd core itself already contains a reference to it or
298 loading the code yourself via <code>dlopen()</code>.</li>
302 <div class="bottomlang">
303 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
304 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
305 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
306 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
307 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
308 </div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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